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Outline• Features of heterogeneous networks• Improvement target in system capacity• Multi-access schemes that improve frequency

efficiency• Adaptive beamforming technology

Efficient Radio Access Technologies for 5G Mobile Communications

M. SawahashiDepartment of Information and Communication

Engineering, Tokyo City University

2020s2010s2000s1990s

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Multi-access Schemes in Cellular Systems

1G 2G 3G 4G

GSMPDC

D-AMPS

W-CDMACDMA2000

FDMA TDMA CDMA

HSDPA/HSUPA

LTEWiMAX

LTE-AdvancedWiMAX-

Advanced

OFDMASC-FDMA

(N)OFDMA(N) SC-FDMA

3.0G 3.5G 3.9G 4.0G

Circuit-switched based access(Dedicated channel) Packet based access

(Shared channel)

CDMA(Code-multiplexing)

Difference in radio access schemes 5G

NewRadio

Access

1980s

OFDMA: Orthogonal Frequency Division Multiple AccessSC-FDMA: Single-Carrier FDMA

Carrier aggregation

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Heterogeneous Networks

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• Efficient radio access networks are needed that can accommodate drastically increasing volume of traffic in cellular systems

• Traffic in cellular networks occurs non-uniformly in special areas such as hotspots, large halls, and underground shopping malls

Heterogeneous networks- Macrocell overlaid onto small cells such as picocells and

femtocells - Small cell effectively supports non-uniform traffic in high

traffic-density areas

Heterogeneous Networks

Small cells (outdoor) Small cells (indoor)

MacrocellHigh traffic density

Heterogeneous networks

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Centralized control(Baseband signaling)

RRERRE

eNodeB

RRE

Macrocell

Small cell (outdoor)

S-GW

Small cell (indoor)

RRERRE

Backhaul/Fronthaul• Optical fiber• Wireless backhaul/

fronthaul

Heterogeneous Networks Structure

MME

• MME: Mobility Management Entity• S-GW: Serving Gateway• RRE: Remote Ratio Equipment

Radio access network • Broadband • Low latency

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Heterogeneous Networks with Carrier Aggregation

Heterogeneous networks employing carrier aggregation (CA) Effective in efficiently increasing system capacity for non-uniform traffic distributions with different QoSs (data rate, latency).

• Macrocell with low frequency Guarantees wider area coverage• Small cell with high frequency Provides broadband services

1 GHz 10 GHz 100 GHz3 GHz 6 GHz

Existing spectrum New spectrum

Carrier aggregation (Spectrum aggregation)

Macrocell

Small cell (outdoor)

High traffic density

Small cell (indoor)

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Improvement Target in System Capacity

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Approaches to Increase System Capacity for 5G Radio Access

Extending available frequency spectra Improving spectral efficiency Densifying BSs using heterogeneous networks

Spectral extension

Improvement in spectral efficiency

BS densification

Nokia 10 10 10

Huawei 4 16-30 10

Docomo 2.8 2.4 15

Ericsson 4 2.5 100

RWHA 3 5 60

Target improvement in spectral efficiency through radio access technologies 3 – 10 times (from LTE-Advanced)

Source: Markus Dominik Mueck, Keynote speech, WPMC2014, Sept. 2014.

Target Peak Data Rate for 5G Radio Access

Increasing peak data rate and peak frequency efficiency Effective in increasing system capacity, cell throughput, and cell-

edge user throughput

Proposals for target peak data rate for 5G radio interface• ARIB: 10 Gbps• Samsung (Korea): 50 Gbps (*)• Huawei: 1 Tbps /cell site (BS), 10 Gbps/ UE (**)

(*) K. Cheun, Plenary talk, IEEE VTC2014-Spring, May 2014.(**) W. Tong, Plenary talk, IEEE VTC2014-Fall, Sept. 2014.

Target peak-data figure for 5G radio access • 10 Gbps – Several 10 Gbps• Smooth and scalable extension to beyond 100 Gbps Achieve extension using antenna space domain and/or frequency

domain based on the same multi-access schemes

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Need to increase the peak data rate and frequency efficiency for packet based radio access.

• Peak data rate of shared channel Several 10s Gbps – 100 Gbps However, according to increases in available frequency bandwidth

and number of antennas,• Increase in control signal overhead restricts achievable performance

of frequency domain scheduling and precoding in MIMO• Increase in reference signal (RS) overhead restricts effective

transmission power and resource

Control signal and reference signal (RS) overhead

Real available resources for carrying user data

Further Improvement in System Capacity

To improve system capacity for 5G radio access Need efficient radio access technologies with low overhead for

control signals and RSs

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Efficient multiplexing of system-specific and cell-specific broadcast information

Dual-connectivity (Splitting of C-plane/U-plane) Phantom cell (*)

• Transmits broadcast information from a macrocell that guarantees wide area coverage

• Transmits broadband service traffic from small cell using wider bandwidth with high carrier frequency

Ultra-lean design for control channel and RS (**), (***)• Minimizes amount of “always-broadcast” system information• Scheduling based RS multiplexing and measurement

(*) 3GPP RWS-120010, DOCOMO, June 2012.(**) 3GPP RWS-120003, Ericsson, June 2012. (***) IEEE Commun. Mag. Feb. 2013.

11UE

U-plane

C-plane

Efficient Multiplexing for Common Control Signals

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Multi-access Schemes that Enhance Frequency Efficiency

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Time

Frequency

Layer(Code, Space)

Type III (Type IV)

Type II

Type I

Unified Frame Structure Unified radio interface is desirable to carry various types of traffic

with different sizes and delay requirements • Mobile broadband (MBB): High-density moving video, streaming,

and downloading of large files.• Machine-Type Communication (MTC): Massive MTC traffic and MTC

traffic with very low latency, i.e., mission-critical MTC

Unified frame structure (*) Multiplexing of different multi-access schemes according to different requirements using FDM /TDM

With scheduling and ATTC (LTE)

With scheduling and without ATTC

MTC

(*) IEEE Commun. Mag., vol. 52, pp. 97-105, Feb. 2014.

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Scheduling or Frequency Diversity MBB Frequency domain channel-dependent scheduling• Large frequency scheduling gain• Large RB size mitigates relative control signal overhead

- Control signals are necessary such as ATTC, scheduling grant, and frequency-selective CQI.

MTC traffic with strict delay requirement Fast transmission without feedback loop

• MTC that requires low power consumptionnarrowband transmission- Intra-subframe frequency hopping (FH)

• MTC traffic with allowable level of power consumption - Spreading with sparse signature (low-rate channel coding)

Frequency

Time• Resource assignment based on CQI

Freq.

Time

FH

Spreading

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Multi-access Schemes for Frequency Spectra Unified radio interface comprising a few multi-access schemes

and radio parameter sets according to system requirements • Frequency spectra: Wide range of spectra from 800 MHz band to

millimeter-wave band• User data rate: From several kbps to several Gbps• Latency: Shorter than 1 ms for mission-critical MTC Small number of options is desirable

Existing and additional spectrum below 6 GHz

Candidate 5G multi-access schemes

LTE based multi-access scheme• OFDMA• SC-FDMA

10 – 30 GHz

OFDM based multi-access scheme Optimized radio parameters• Short TTI (for MTC)• Non-orthogonal

multi-access

Millimeter-wave bands

Single-carrier based multi-access scheme- High path loss- To mitigate

implementation requirement for RFcircuitry

Source: M. Latva-aho, Keynote speech, WPMC2014, Sept. 2014.

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Major Radio Parameters in LTE

FFT block (Effective OFDM symbol) CP Subcarrier spacing: fsc= 1/TFFT

Narrower subcarrier spacing (Longer FFT block length)

• Greater influence of phase noise of VCO

TTI (Transmission Time Interval) of 1 ms

Tradeoff relation

Subcarrier spacing of 15 kHz (FFT block length of 66.67 s)

Longer TTI length • Longer round trip delay

with hybrid ARQ

Shorter TTI length • Higher ratio of control

signal overhead

Subframe (= 1 ms)

Wider subcarrier spacing(Shorter FFT block length)

• Greater insertion loss of CP

• Greater influence of channel variation during FFT block

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Decrease in transmission delay (latency) Shorter TTI length to achieve shorter round trip delay

- Shorter FFT block- Fewer FFT blocks within a TTI

Decrease in overhead of control signals and RS within 2-dimensional RB in time and frequency domains

Numerology(*), (**),(***)

- Shorter TTI length: 0.1 - 0.2 ms- Subcarrier spacing: Integer times that of LTE (fs,NX = fs,LTE x M/N ,

with reasonable M, N) - Shorter CP duration

(*)RWS-150009, (**)RWS-150039, (***)RWS-150051

Design Proposals for 5G Radio Parameters

FFT blockCP FFT block

CP

Freq. Freq.

• Shorter FFT block

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DL Multi-access Schemes

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• Decrease guard bands on both sides of assigned frequency spectrum

• Efficient multiplexing of RBs with different subcarrier spacing, i.e., FFT block length, to achieve short delay Decrease guard bands between RBs with different subcarrier spacing

Suppress side lobes outside assigned RB through spectral shaping- FBMC (Filter Bank Multicarrier): Subcarrier-level spectral shaping- UF (Universal Filtered)-OFDM: RB level spectral shaping

associated with IDMA- GFDM (Generalized Frequency Division Multiplexing): Pulse

shaping filters at transmitter and receiver using tail biting

Reducing Adjacent Channel Leakage Power (ACLP) Level

Tim

e

FrequencyIntra-system guard band

Inter-system guard band

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Efficient Multiplexing of Small Size MTC Traffic

Non-orthogonal multiplexing of massive MTC: Combination with CDM using LDS (Low Density Signature) is effective in increasing number of channels with low data rate

Techniques that ontain frequency diversity effect without feedback loop- Spreading- Distributed transmission- Intra-subframe Frequency hopping (FH)

Small impact on control signal overhead

Time

Frequency

Ex. 1 FFT block / TTI

1 FFT block, TTI

Wideband transmission

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Non-orthogonal multi-access (non-orthogonal multiplexing, i.e., high-density resource packing)

Requirements Commonality with existing orthogonal multi-access schemes

• Same radio parameters• Same radio access technologies are used• Transparent from higher layer, i.e., same multiplexing scheme for

transport channel as in orthogonal multi-access Coexistence with orthogonal multi-access schemes in the same

frequency spectrum is desirable

Further Enhancement in Frequency Efficiency

Frequency

Time

Resource for orthogonal access

Resource for non-orthogonal access

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Tim

e ･･･

T

Frequency

･･･

･･･

Data symbol

ISI

T

･･････

･･･ ･･･

Tim

e

Data symbol

･･･

Frequency

T･･･

･･････

(User #2)

･･･

(User #1)

Data symbol (User #1)

T

T: FFT block length

Orthogonal multiplexing among different UEs

Non-orthogonal multiplexing among

different UEsFaster-than-Nyquist (FTN)

signalingSuperposition coding

(NOMA)ISI and ICI Occurs Does not occur

CP Not used UsedNotification of control

signals among different UEs

Not requiredRequired (transmit

power ratio, precodingvector)

Non-orthogonal Multi-access Scheme Candidates

Orthogonal RB

Non-orthogonal RB

Orthogonal RB

Non-orthogonal

RB

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UL Multi-access Schemes

MBB: Synchronous orthogonal or non-orthogonal multi-access scheme

• Adaptive Transmit Timing Control (ATTC) aligns received timings of simultaneously accessing users within CP duration (used in LTE)

Best multiplexing scheme for large size traffic without strict delay requirement for DFT-Spread OFDM

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eNodeB UECS-RS, Common control signals

RACH

Transmit timing control signal

UL shared channel (user data)

Synchronous Reception for MBB Traffic

Time

Guard Interval(Cyclic prefix duration)

Subframe

RB

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Mission-critical IoT for MTC• Very short delay requirement • Small size of massive IoT Low control signal overhead

Open-loop synchronization based on received timing of DL signal• Shared channel transmission in UL synchronized with DL received

timing received timing error remains due to round-trip propagation delay

• Influence of received timing error is mitigated by pulse shaping filter such as FBMC, UF-OFDM, or GFDM

eNodeB UE

CS-RS, Common control signals

UL shared channel (user data)

Asynchronous Reception for MTC

TimeGuard Interval(Cyclic prefix duration)

Subframe

RB

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Efficient Multiplexing of MTC Traffic

FrequencyPhysical channel

(user) index

Different color indicates different signature

• Small size MTC with very short delay requirement Multiplexing without feedback-loop is suitable

Spreading using Low Density Signature (LDS)[1] Frequency diversity effect is obtained by spreading using sparse signature- LDS-CDMA and LDS-OFDM[2]: Symbol mapping + Spreading based

on LDS associated with MPA (Message Passing Algorithm)- SCMA (Sparse Code Multiple Access)[3]: Direct spreading from

information bit to sparse codeword[1] R. Hoshyar, F. P. Wathan, and R. Tafazolli, IEEE Trans. Signal Process., vol. 56, pp. 1616-1626, Apr. 2008. [2] R. Hoshyar, R. Razieh, and M. AL-Imari, Proc. IEEE VTC2010-Spring, May 2010. [3] H. Nikopour and H. Baligh, Proc. IEEE PIMRC2013, Sept. 2013.

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Adaptive Beamforming Technology

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• Beamforming gain extends coverage for high frequency spectrum• Increasing data rate by spatial division multiplexing (SDM) Massive MIMO with beamforming and spatial multiplexing gains

Decrease in overhead of RS and control signals is necessary RS: CSI-RS, demodulation RS Control signals: CSI, CQI, RI, PMI (feedback signals), scheduling

grant, modulation index, and TB size

TDD (Time Division Duplex) is more advantageous compared to FDD (Frequency Division Duplex) for reducing RS and control signal overheads.

- Feedback signals are not needed when using channel reciprocity

Multi-antenna Transmission with Beamforming

CSI (CQI) measurement in ULBeamforming transmission in DL

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TDD FDDCriterion to generate or

select directive beam

Maximize instantaneous

SINR

Maximizeaverage

SINR

Maximize instantaneous

SINR

Maximizeaverage

SINR

Beamforming gain High Lower High Lower

Frequency-selective

precodingUse Do not use Use Do not use

CQI feedback and CSI-RS overhead

Do not needCQI/

CSI-RS (high density)

Do not needCQI/ CSI-RS (low density)

Subbandbased CQI

Wideband CQI

RF circuitry calibration Need Need Do not need Do not need

Comparison of Adaptive Beamforming Schemes

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Adaptive beamforming method• Criterion based on maximum average SINR is better than that based

on maximum instantaneous SINR from viewpoint of decreasing control signal overhead and control delay

• However, additional diversity is required to compensate for decreasing beam forming gain Frequency diversity etc.

Adaptive Beamforming Method

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Frequency

SubframeSlot #0

Slot #1

Adaptive beamforming with wideband precoding

Frequency diversity techniques• Channel-dependent scheduling• Spreading • Intra-subframe FH

Tim

e

Spreading

Intra-subframeFH

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Conclusion

Presented efficient multiplexing schemes for control signals and non-orthogonal multi-access schemes for shared channel based on unified frame structure.